OSPF Type 7 LSA
An OSPF Type 7 LSA is used in Not So Stubby Areas (NSSAs) to allow the injection of external routes from an ASBR router while not allowing any other external routes into the stub area.
The purpose of an NSSA is to allow an ASBR to share its external routes into the stub area without allowing any other external routes to be shared via the ABR.
For this reason, no Type 5 LSAs are allowed within the NSSA. But this disallows an ASBR from sharing its external routes. Therefore, another LSA type, Type 7 was introduced that can be generated only by ASBRs.
Take a look at this network topology:
R1 is an ASBR within an NSSA. Type 5 LSAs are not allowed in such an area. So the ASBR sends Type 7 LSAs.
How does this benefit an NSSA as far as efficiency goes? Well, R2 (the ABR) is still restricted from sending any Type 5 LSAs into Area 2. Therefore, R2 will only send a default route, so efficiency is still maintained by disallowing any detailed routes from being advertised into the NSSA from the ABR.
Links:
https://forum.networklessons.com/t/nssa-purpose/45617/5?u=lagapidis
https://networklessons.com/ospf/how-to-configure-ospf-nssa-not-so-stubby-area
https://networklessons.com/ospf/introduction-to-ospf-stub-areas